The idea that bigger, more cognitively adept brains evolved as primates began living in larger, more socially complex groups—the social brain hypothesis—was recently put to the test by anthropologists Alex R. DeCasien, Scott A. Williams, and James P. Higham at New York University and the New York Consortium in Evolutionary Primatology, in the most comprehensive study thus far.

Results on brain size in previous studies conflicted with each other, only sometimes supporting the social brain hypothesis. Some research had also suggested that monogamous primate species had bigger brains, while other research suggested that big brains were associated with promiscuity. “Resolving that contradiction was our initial goal,” says DeCasien.

From large, existing datasets, the team used brain and body size information for more than 140 primates, representing a broad array of Old World and New World monkeys, apes, and lemurs (humans were excluded because our social and mating systems are diverse and highly variable). An analytical model, incorporating evolutionary ancestry from phylogenetic analyses, included such factors as mating system, social system, group size, and diet quality in order to test the relative importance of these elements in predicting brain size, while controlling for phylogeny.

Their model indicated that diet—whether species are frugivores (fruit eaters), folivores (leaf eaters), or omnivores—best explains brain size. Frugivores tend to have larger brains relative to body size than folivores. Measures of sociality, in contrast, explained none of the brain size variation. This new work supports earlier hypotheses positing that, because fruit is patchily dispersed in time and space, it requires more advanced spatial memory to find, providing a selective force for larger brains. Additionally, fruit often needs to be extracted from protective skins. “If you think of the complex manipulative abilities it might take to peel an orange, for example, selection on increased manipulative ability would also drive frugivores to have larger brains,” DeCasien says.

The research team acknowledges that overall brain size may be a rough proxy for cognitive complexity, with others proposing a better measure is the size of the brain’s neocortex region. However, a similarly comprehensive dataset for neocortex size is not yet available. The researchers suggest that selection for increased foraging efficiency may have provided “the scaffolding for subsequent development of social skills.” (Nature Ecology & Evolution)